Meiosis has evolutionary significance because it results in
(a) genetically similar daughters
(b) four daughter cells
(c) eggs and sperms
(d) Recombination takes place in meiosis but still Meiosis maintains the chromosome number constant. It produces haploid gametes by reducing the chromosome number to half. Crossing over produces new combination of linked genes and is major source of genetic variation.
Also, distribution of bivalents which is at random in metaphase-I provides the secondary source of genetic variation in the organisms and is essential for speciation and evolution.
A bivalent consists of
(a) two chromatids and one centromere
(b) two chromatids and two centromeres
(c) four chromatids and two centromeres
(d) four chromatids and four centromeres
(c) Each pair of homologous chromosome carrying one maternal and one paternal chromosome of similar type is called bivalent. Each chromosome has two sister chromatids and a centromere. Thus, bivalents possesses four chromatids, two centromeres. This bivalent with four chromatids is called pachytene tetrad (quadrivalent).
Bacterium divides every 35 minutes. If a culture containing 105 cells per mL is grown for 175 minutes, what will be the cell concentration per mL after 175 minutes?
(a) 5 x 105 cells
(b) 35 x 105 cells
(c) 32 x 105 cells
(d) 174 x 105 cells
(c) 1 x 105 2 x 105 4 x 105
8 x 105 16 x 105 32 x 105
During cell division, the spindle fibres attach to the chromosomes at a region called
(b) During late prophase, specialised structures called kinetochores develop on either surface of the centromere. Chromosomal fibres get attached to kinetochore.
Chromomeres are beaded structures on the chromosomes which are found particularly at the prophase-I (particularly at leptotene) of meiosis-I.
Chromocentre is developed due to the fusion of centromeric regions of all the chromosomes of a cell. Centrosome is found in animal cells(absent in higher plant cells). It is found near the nucleus. Each centrosome is made up of two centrioles and each centriole is a cylindrical structure with a diameter of 1500-1800 and is made up of nine fibrils.
What would be the number of chromosomes of the aleurone cells of a plant with 42 chromosomes in its roots tip cells?
(a) 21 (b) 42 (c) 63 (d) 84
Aleurone layer is the outermost cell layer of the endosperm. The ploidy of the endosperm is triploid. If the root cell which is diploid has 42 chromosomes, so the haploid cell would have 21 chromosomes and triploid would have 3 X 21=63 chromosomes.
Synapsis occurs between
(a) a male and a female gamete
(b) mRNA and ribosomes
(c) spindle fibres and centromere
(d) two homologous chromosomes
(d) In zygotene of prophase-I, homologous chromosomes pair up. This proces is called synapsis. One chromosome of the pair is from the male parent and one from the female parent.
Best stage to observe shape, size and number of chromosomes is
(b) Metaphase can be characterised by the chromosomes that are least coiled which show maximum condensation and are shortest in length. It is the best stage to study the structure, size and number of chromosomes in a cell. Idiogram/karyotype of chromosomes is prepared at metaphase.
If a diploid cell is treated with colchicine then it becomes
(a) triploid (b) tetraploid
(c) diploid (d) monoploid
(b) Cholchicum autumnale provides an alkaloid called colchicine which is used in plant breeding for doubling the chromosome number. Treatment with 0.1% colchicine inhibits spindle formation so that chromatids fail to separate during anaphase.
Which one of the following precedes reformation of the nuclear envelope during M-phase of the cell cycle?
(a) Decondensation from chromosomes and reassembly of the nuclear lamina
(b) Transcription from chromosomes and reassembly of the nuclear lamina
(c) Formation of the contractile ring and formation of the phragmoplast
(d) Formation of the contractile ring and transcription from chromosomes.
(a) At telophase stage, nuclear membrane vesicles associate with the surface of individual chromosomes and fuse to reform the nuclear membranes, which partially enclose clusters of chromosomes before coalescing to reform the complete nuclear envelope. During this process the nuclear pores reassemble and the dephosphorylated reassociate to form the nuclear lamina. One of the lamina protiens (lamina-B) remains with the nuclear membrane fragments throughout mitosis and may help nuclear reassembly. After the nucleus reforms, the pores pump in nuclear protiens, the chromosomes decondense and RNA synthesis resumes, causing the nucleolus to reappear.
During mitosis ER and nucleolus begin to disappear at
(a) late prophase
(b) early metaphase
(c) late metaphase
(d) early prophase
(d) In mitosis, prophase is the longest phase of karyokinesis. In early prophase, nuclear membranes, nucleolus start disintegrating. Cells cytoskeleton, Golgi complex, ER, etc, disappear.